Freeciv

I guess FC_FREE() is supposed to be handy, and even useful tool for making sure freed pointers get nullified.

I don't like it.
Problems include:
- It evaluates its parameter twice (e.g. risk of "FC_FREE(array[index++])", among others)
- It gives false sense of security, when there's some more relevant pointer to nullify than one to pass to FC_FREE()
- In some cases it prevents analysis tools from detecting errors they would otherwise detect, or to produce less accurate info

It's also not consistently used, but plain free() is more common in the freeciv codebase. I suggest we get rid of it. This is a metaticket towards that goal.